Method of forming passive oxide film based on chromium oxide, and stainless steel

ABSTRACT

A method of readily forming passive oxide film based on chromium oxide characterized by subjecting stainless steel to electrolytic polishing and fluidized abrasive polishing, baking the steel thus treated in an inactive gas to remove moisture from its surface, and heat treating the resultant steel at 300° to 600° C. in a gaseous atmosphere comprising hydrogen or a mixture with an inactive gas and containing less than 4 ppm of oxygen or less than 500 ppb of moisture. An oxidized stainless steel characterized by comprising a stainless steel having a crystal grain number of 6 or above and, formed on the surface thereof, a passive oxide film based on chromium oxide, wherein the oxide film has a thickness of 5 nm or above and the atomic ratio of chromium to iron in the outermost layer of the film is 1 or above.

TECHNICAL FIELD

The present invention relates to a method for forming a passive oxidefilm having chromium oxide as a chief component thereof, as well as to astainless steel.

BACKGROUND OF THE INVENTION

Conventionally, two methods were known for the formation of a passiveoxide film having chromium oxide as a chief component thereof on astainless steel surface: the dry method, in which, after directlyreacting stainless steel with oxygen gas, the oxidized steel was reducedwith hydrogen gas and heat treated with an inert gas such as argon afterreduction, and thereby, a passive film having chromium oxide as a chiefcomponent thereof was formed; and the wet method, in which the steel wasetched using a chemical such as nitric acid or the like, and chromiumoxide was obtained. A diagram of the processes of the dry method isshown in FIG. 5(b).

In FIG. 5(b), (1) indicates a baking process which removes moistureadhering to the stainless steel surface, and moisture released by thestainless steel surface. (2) indicates an oxidation process which isconducted in an oxygen atmosphere. The film obtained by this oxidationprocess is a passive oxide film having iron oxide as a chief componentthereof. (3) indicates a reducing process in which the iron oxide isreduced in a hydrogen atmosphere in order to obtain chromium oxide. (4)indicates a heat treatment process in an inert gas atmosphere for thepurpose of conversion to a film having chromium oxide as the chiefcomponent thereof. In this way, in accordance with the dry method, theformation of the chromium oxide is conducted by means of independentoxidation and reduction reactions, so that the period required for theprocesses is long.

FIG. 6 shows data relating to moisture released at normal temperaturesfrom passive oxide films obtained by means of the wet method and the drymethod, as measured by APIMS. As is clear from FIG. 6, in contrast tothe passive oxide film formed in accordance with the dry method, whichceased giving off moisture after several minutes, the passive oxide filmobtained in accordance with the wet method continued to give offmoisture even after the passage of 100 minutes. In this way, the passiveoxide film obtained in accordance with the wet method contained a largemoisture component, so that if the moisture were not removed, such apassive oxide film could not be used in semiconductor productionapparatuses, which must be free of outside gasses, and heat treatmentsuch as baking or the like was necessary, so that in the same manner aswith the dry method, considerable time was required.

The present invention has as an object thereof to provide a method offorming a passive oxide film having chromium oxide as a chief componentthereof which is capable of easily forming a passive oxide film havingchromium oxide as a chief component thereof, and to provide a stainlesssteel having a passive oxide film having chromium oxide as a chiefcomponent thereof.

SUMMARY OF THE INVENTION

A first essential feature of the present invention resides in astainless steel having a crystal grain number of 6 or above and havingformed on the surface thereof a passive oxide film having a thickness of5 nm or above and in which the value of Cr/Fe (hereinbelow, this refersto an atomic ratio) at the outermost layer of the film is 1 or above.

A second essential feature of the present invention resides in astainless steel having an amount of warp of 0.2% or more having formedon the surface thereof a passive oxide film having a thickness of 5 nmor above, and wherein the value of Cr/Fe at the outermost layer of thefilm is 1 or above.

A third essential feature of the present invention resides in a methodof forming a passive oxide film having chromium oxide as a chiefcomponent thereof, characterized in that stainless steel is subjected toelectrolytic polishing, then baking is conducted in an inert gas, andthereby, moisture is removed from the surface of the stainless steel,and then heat treatment is conducted at a temperature within a range of300° C. to 600° C. in a gaseous atmosphere comprising hydrogen or amixture thereof with an inert gas and containing less than 4 ppm ofoxygen or less than 500 ppb of moisture.

A fourth essential feature of the present invention resides in a methodof forming a passive oxide film having chromium oxide as a chiefcomponent thereof, characterized in that stainless steel is subjected tocomposite electrolytic polishing, then baking is conducted in an inertgas, and thereby, moisture is removed from the surface of the stainlesssteel, and then heat treatment is conducted at a temperature within arange of 300° C. to 600° C. in a gaseous atmosphere comprising hydrogenor a mixture thereof with an inert gas and containing less than 4 ppm ofoxygen or less than 500 ppb of moisture.

A fifth essential feature of the present invention resides in a methodof forming a passive oxide film having chromium oxide as a chiefcomponent thereof, characterized in that a stainless steel is subjectedto fluidized abrasive polishing, then baking is conducted in an inertgas to remove moisture from the surface of the stainless steel, and thenheat treatment is conducted at a temperature within a range of 300° C.to 600° C. in a gaseous atmosphere comprising hydrogen gas or a mixturethereof with an inert gas and containing less than 4 ppm of oxygen orless than 500 ppb of moisture.

Hereinbelow, the function of the present invention will be explainedtogether with embodiment examples.

It is preferable that SUS316L having a composition such that, forexample, C≦0.020% (hereinbelow, this percentage refers to weightpercent), Si≦0.50%, Mn≦0.80%, P≦0.030%, S≦0.0020%, Ni is within a rangeof 12.0%-17.0%, Cr is within a range of 17.0%-24.0%, Mo is within arange of 0.05-3.5%, and Al≦0.020%, be used as the stainless steel whichis the object of the present invention. It is preferable that the amountof oxygen contained be 20 ppm or below, and an amount less than severalppm is further preferable. If the amount of oxygen contained exceeds alevel of 20 ppm, a porous passive film will be formed, and a porouspassive film exhibits low resistance to corrosion even if the Cr/Feratio is high.

In the method of the present invention, the stainless steel is firstsubjected to electrolytic polishing. The surface roughness afterelectrolytic polishing should, from the point of view of the formationof a minute passive film, be 5 μm or less and a roughness of 1 μm orless is further preferable, while a roughness of 0.5 μm or less is stillfurther preferable.

After electrolytic polishing, baking is conducted in an inert gas, andthereby, moisture present on the surface of the stainless steel isremoved. The baking temperature and period are not particularly limited,if as the temperature is sufficient to remove adhering moisture;however, a temperature within a range of, for example, 150° C. -200° C.is acceptable. The baking should preferably be conducted in an inert gas(for example, Ar, or N₂) atmosphere having a moisture content of lessthan several ppm.

Next, heat treating is conducted at a temperature within a range of 300°C. -600° C. in a gaseous atmosphere comprising hydrogen or a mixturethereof with an inert gas and containing less than 4 ppm of oxygen orless than 400 ppb of moisture At temperatures of less than 300° C. theformation of a passive film having chromium oxide as a chief componentthereof is insufficient. When the temperature exceeds 600° C. theminuteness of the passive film which is formed is poor. A temperaturerange of 400° C.-600° C. is further preferable for this heat treatment.The period of heat treatment should preferably be within a range of from10 minutes to less than 10 hours, and a period within a range of 30minutes to less than several hours is further preferable.

In the present invention, it is preferable that a stainless steel havinga crystal grain size of 6 or more be used, and it is further preferablethat a stainless steel having a crystal grain size of 8 or above beused. When a stainless steel having such a grain size is used, theatomic range of Cr/Fe at the surface of the passive film which is formedincreases greatly. The reason for this is somewhat unclear; however, itis thought that when stainless steel having this crystal grain size isused, the chromium atoms are dispersed throughout the surface via thecrystal grain boundaries, so that the value of Cr/Fe increases greatly.

When a stainless steel having a grain number of 6 or more is used duringformation of the passive oxide film by means of high temperature bakingat a temperature within a range of 400° C.-600° C. in an inert gasatmosphere after electrolytic polishing, the thickness of the passivefilm increases, and furthermore, it is possible to form a passive filmhaving chromium oxide as the chief component thereof.

Furthermore, in place of regulating the crystal grain size of thestainless steel, it is possible to conduct cold working having a surfacereduction ratio of 2% or more prior to electrolytic polishing.

When stainless steel having an oxygen content of several ppm or below isemployed, it is possible to form a passive film which is more minutethan that formed in the case of stainless steel having an oxygen contentof several ppm or more.

If composite electrolytic polishing or fluidized abrasive polishing isconducted in place of electrolytic polishing, it is possible to form apassive film which is minute and has a high Cr content. That is to say,the passive oxide film which is formed on the surface of the stainlesssteel contains a higher concentration of chromium oxide and is a moreminute film than that formed in the case in which electrolytic polishingis conducted. The reason for this is thought to be that microfissuresare generated on the surface as a result of composite electrolyticpolishing or fluidized abrasive polishing, and chromium is deposited inthe surface through these fissures. Such fissures are either covered bythe passive film during passive film formation, or are eliminatedthereby, and thus do not affect the surface characteristics.

It is still further preferable that after composite electrolyticpolishing or fluidized abrasive polishing, a slight electrolyticpolishing be conducted in order to remove the layer altered by working,and that the surface layer be etched to a depth of several molecules.

Furthermore, in the present invention, if the stainless steel is heatedin a gaseous atmosphere comprising hydrogen or a mixture of hydrogen gasand an inert gas (for example, argon gas or nitrogen gas) afterconducting electrolytic polishing, composite electrolytic polishing, orfluidized abrasive polishing, oxygen from a porous layer containingoxygen which remains on the surface of the stainless steel afterelectrolytic polishing serves as a source of oxygen for formation of thepassive film, and as described above, the oxidation and reductionreactions occur simultaneously, and a passive oxide film having chromiumoxide as a chief component thereof can be easily formed by reducing theiron oxide. The amount of oxygen contained in the stainless steel maypreferably be within a range of from several ppm to 1 weight percent orbelow. In this case, as well, it is preferable that compositeelectrolytic polishing or fluidized abrasive polishing be conducted, andit is further preferable that after this, slight electrolytic polishingbe conducted and the surface be etched to a depth of several molecules.

Hereinbelow, the present invention will be explained in detail.

In the present invention, as shown in FIG. 5(a), simply by conductingthe baking process and the oxidation and reduction process, it ispossible to form a passive oxide film having chromium oxide as a chiefcomponent thereof.

In the formation method for the passive oxide film having chromium oxideas the chief component thereof in accordance with the present invention,first, the surface of the stainless steel is subjected to electrolyticpolishing. It is preferable that the surface roughness thereof be Rmax 5μm or less. Next, baking is conducted, and thereby the adhering moistureis removed.

Next, the stainless steel is subjected to heat treatment in the presenceof hydrogen containing a trace amount of oxygen or a trace amount ofmoisture. Simply by conducting such heat treatment, a passive oxide filmhaving chromium oxide as a chief component thereof is formed. In thiscase, less than 4 ppm of oxygen or less than 500 ppb of moisture shouldbe present.

In contrast, in the case in which stainless steel is employed whichcontains oxygen, there is no need to externally supply oxygen ormoisture.

The hydrogen may be diluted with an inert gas, and it is preferable thatthe hydrogen concentration be within a range of from less than severalppm-10%.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an XPS analysis of the passive oxide film formed inEmbodiment 1.

FIG. 2 shows an XPS analysis of the passive oxide film formed inEmbodiment 2.

FIG. 3 shows an XPS analysis of the passive oxide film formed in aComparative Example.

FIG. 4 shows an XPS analysis of the passive oxide film formed inEmbodiment 3.

FIG. 5(a) is a process diagram showing the processes for formation of apassive film in accordance with the method of the present invention, and

FIG. 5(b) is a process diagram showing the conventional processes forpassive film formation.

FIG. 6 is a graph showing data relating to moisture released frompassive oxide films at normal temperatures as measured by APIMS.

FIG. 7 shows an XPS analysis of the passive oxide film formed inEmbodiment 4.

FIG. 8 shows an XPS analysis of the passive oxide film formed inEmbodiment 4 after a corrosion resistance test.

FIG. 9 is a scanning electron micrograph of the passive oxide filmformed in Embodiment 4 after the corrosion resistance test.

FIG. 10 shows an XPS analysis of the passive oxide films formed afterwelding and formed at the welded portion.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Hereinbelow, the present invention will be explained in further detailbased on Embodiments.

(Embodiment 1)

In the present Embodiment, SUS316L stainless steel having a grain numberof 5 and containing 25 ppm of oxygen was subjected to electrolyticpolishing, and a surface roughness of approximately 5 μm was obtained.

Next, the stainless steel was placed in a furnace, and baking wasconducted at 150° C. for a period of 2 hours while supplying an Ar gashaving an impurity concentration of less than ppb into the furnace, andmoisture adhering to the surface was removed.

After the completion of the above baking, hydrogen gas was mixed withargon gas so that a hydrogen concentration of 10% was reached, and heattreatment was carried out at a temperature of 500° C. and for a periodof 2 hours.

The results of an XPS analysis of the passive film formed under theabove conditions are shown in FIG. 1. The sputtering rate was 10 nm/min.As is clear from FIG. 1, the concentration of the chromium component washigh to a considerable depth in the passive film formed under the aboveconditions, and it is clear that a passive film having chromium oxide asa chief component thereof was formed. That is to say, the value of Cr/Feis 5 or greater, and the thickness of the passive film was 2.5 nm orgreater.

(Embodiment 2)

In the present Embodiment, stainless steel (SUS316L) in which the oxygencontent was maintained at a level of less than several ppm was employed.

The other conditions were identical to those of Embodiment 1, andelectrolytic polishing and baking were conducted.

However, heat treatment was conducted at a temperature of 500° C. andfor a period of 1 hour in a gas in which hydrogen and oxygen were addedto an argon gas base so that the hydrogen concentration was 10%, andoxygen was present at a level of 100 ppb.

The results of an XPS analysis of the passive film formed under theabove conditions are shown in FIG. 2. As is clear from FIG. 2, thepassive film formed under the above conditions was a passive film havingchromium oxide as a chief component thereof. That is to say, the valueof Cr/Fe was 6 or greater, and the thickness of the passive film was 5nm or greater.

(Comparative Example 1)

In the present Comparative Example, as in Embodiment 2, stainless steelhaving an oxygen content of several ppm or below was employed.Furthermore, electrolytic polishing and baking were conducted in amanner identical to that of Embodiment 2.

Next, heat treatment was conducted at a temperature of 500° C. and for aperiod of 1 hour in a mixed gas in which hydrogen and oxygen were addedto an argon gas base so that the concentration of hydrogen was 10% andthe concentration of oxygen was 10%.

The results of an XPS analysis of the passive film formed under theabove conditions are shown in FIG. 3. As is clear from FIG. 3, thepassive film has iron oxide as a chief component. It can be seen that ifthe amount of oxygen added exceeds the appropriate amount, the iron isnot reduced but is oxidized.

(Embodiment 3)

In the present. Embodiment, heat treatment was conducted at atemperature of 500° C. and for a period of 1 hour in a gas in whichhydrogen, oxygen, and moisture were added to an argon gas base so thatthe concentration of hydrogen was 10%, oxygen was present at a level of100 ppb, and moisture was present at a level of 100 ppb. The otherconditions were identical to those in Embodiment 2.

The results of an XPS analysis of the passive film formed under theabove conditions are shown in FIG. 4. As is clear from FIG. 4, thepassive film formed under the above conditions has chromium oxide as achief component thereof. That is to say, the value of Cr/Fe is 5 orgreater, and the thickness of the passive film was 5 nm or more.

(Embodiment 4)

Using SUS316L stainless steel, electrolytic polishing was conducted in amanner identical to that of Embodiment 1. This was designated sample 1.

Next, baking was conducted in a manner identical to that of Embodiment1, heat treatment was conducted at a temperature of 500° C. and for aperiod of 1 hour in an atmosphere of a gas in which hydrogen and oxygenwere added to an argon gas base so that the hydrogen concentration was10%, and oxygen was present at a level of 100 ppb, and a passive oxidefilm was thus formed. This was designated sample 2.

SUS316L stainless steel was subject to composite electrolytic polishing,electrolytic polishing was conducted so as to remove the layer alteredby working on the surface, and baking and heat treatment were conductedin a manner identical to that of sample 2, and a passive oxide film wasformed. This was designated sample 3.

The results of an XPS analysis of the surface layers of samples 1, 2,and 3 are shown in FIG. 7(a), (b), and (c), respectively. As shown inFIG. 7, oxide films having a high concentration of chromium at thesurface were formed on each of samples 1, 2, and 3. However, bycomparing the peak positions of chromium oxide in the XPS spectra, itwas determined that in contrast with the chromium oxide of samples 2 and3, which was a stoichiometric compound, the peak of the chromium oxideof sample 1 represented a shift from the chromium oxide peak in astoichiometric ratio, and it is thus clear that the oxide film presentafter electrolytic polishing is not a minute oxide film. Furthermore,the passive oxide film of sample 3 was not merely thick, but thechromium oxide concentration thereof was extremely high, and moreover,no iron was present within 2 nm of the surface, so that this suggeststhat an extremely minute passive film was formed.

Next, samples 1 through 3 were placed in an extremely harsh environmentof HCl gas at a temperature of 100° C. for a period of 20 minutes, andthe state of the surface was then observed by means of a scanningelectron microscope (SEM), and an XPS analysis of the surface layer wasconducted. The results of the XPS analysis are shown in FIG. 8, whilethe scanning electron micrographs are shown in FIG. 9.

As is clear from FIGS. 8 and 9, in sample 1, the chromium concentrationwas greatly reduced, and the surface was rough. This is thought to bebecause the chromium oxide was not stoichiometric chromium oxide, whichhas a high resistance to corrosion. Furthermore, in sample 2, thethickness of the film having chromium oxide as a chief component thereofwas reduced even though the chromium oxide was in a stoichiometricratio, and the chromium concentration at the surface was reduced.Furthermore, slight roughness was observed in the surface. The reasonfor this is thought to be that since iron oxide was contained in largeamounts, the iron oxide separated as a result of corrosion, and thechromium oxide separated along with this. However, a passive film havingchromium oxide as a chief component thereof remained on the surface ofsample 2, and in consideration of the testing conditions, the passivefilm would sufficiently stand up to use under normal conditions.

In contrast to samples 1 and 2, in sample 3, almost no change wasobserved in the surface state and in the film composition before andafter corrosion testing, and thus extremely superior resistance tocorrosion was exhibited. As can be seen from FIG. 7(c), the value ofCr/Fe in sample 3 was 30 or more, and furthermore, the thickness of thesample film was 8 nm or more.

From the above results, it can be seen that a more superior passive filmcan be obtained when composite electrolytic polishing is conducted thanwhen electrolytic polishing is conducted.

(Embodiment 5)

SUS316L stainless steel was subjected to fluidized abrasive polishingusing alumina having a grain size of 20 μm, and then the layer alteredby working was removed from the surface by means of electrolyticpolishing. Next, baking was conducted in a manner identical to that ofEmbodiment 1, and heat treatment was conducted at a temperature of 500°C. and for a period of 1 hour in an atmosphere of a gas in whichhydrogen and oxygen were added to an argon gas base so that the hydrogenconcentration was 10% and oxygen was present at a level of 100 ppb, anda passive oxide film was thus formed.

The passive oxide film which was obtained exhibited extremely superiorresistance to corrosion, as was the case with sample 3 of Embodiment 4.

(Embodiment 6)

SUS316L stainless steel was subjected to composite electrolyticpolishing, and baking was conducted in a manner identical to that ofEmbodiment 1, heat treatment was conducted at a temperature of 500° C.and for a period of 1 hour in an atmosphere of a gas in which hydrogenand oxygen were added to a base argon gas so that the hydrogenconcentration was 10% and oxygen was present at a level of 100 ppb, anda passive oxide film was formed.

The passive oxide film which was obtained had a chromium oxide layer ata depth of 1-2 nm at the surface which was identical to that of sample 3of Embodiment 4. Furthermore, when the corrosion resistance testdiscussed in Embodiment 3 was conducted, slight surface roughness wasobserved. However, as described above, in consideration of theconditions of the corrosion resistance test, the passive oxide film ofEmbodiment 6 would be sufficiently able to stand up to use under normalconditions.

(Embodiment 7)

SUS316L stainless steel was subjected to fluidized abrasive polishingusing alumina having a grain size of 20 μm, and then baking wasconducted in a manner identical to that of Embodiment 1, heat treatmentwas conducted at a temperature of 500° C. and for a period of 1 hour inan atmosphere of a gas in which hydrogen and oxygen were added to a baseargon gas so that the hydrogen concentration reached 10% and oxygen waspresent at a level of 100 ppb, and a passive oxide film was formed.

The passive oxide film which was formed had a chromium oxide layer to adepth of 1-2 nm from the surface which was identical to that of sample 3of Embodiment 4; however, when the corrosion resistance test ofEmbodiment 3 was conducted, slight surface roughness was observed.However, as described above, in consideration of the conditions of thecorrosion resistance test, the passive oxide film of Embodiment 7 wouldbe able to sufficiently stand up to use under normal conditions.

(Embodiment 8)

The interior of a SUS316L stainless steel pipe was subjected tocomposite electrolytic polishing, then a layer altered by working wasremoved from the surface thereof by electrolytic polishing, baking wasconducted in a manner identical to that of Embodiment 1, heat treatmentwas conducted at a temperature of 500° C. and for a period of 1 hour inan atmosphere of a gas in which hydrogen and oxygen were added to a baseargon gas so that the hydrogen concentration reached 10% and oxygen waspresent at a level of 100 ppb, and a passive oxide film was thusobtained.

Next, the stainless steel pipe on which the above passive oxide film wasformed was subjected to welding by means of tungsten inert gas welding,the welded portion was heated to a temperature of 500° C., a gascomposed of an argon base gas to which hydrogen and oxygen were added sothat the hydrogen concentration was 10% and oxygen was present at alevel of 1 ppm, was supplied to the interior of the pipe for a period of1 hour, and the thermal oxidation treatment of the welded portion wasthus conducted.

After this, the pipe was severed and an XPS analysis of the weldedportion was conducted. The results thereof are shown in FIG. 10. Apassive film having an extremely high chromium oxide concentration wasformed at the surface of the welded portion as well, although the reasonfor this is presently unclear.

(Embodiment 9)

In the Embodiment 9, stainless steels were employed having grain numbersof, respectively, 5, 6, 7, and 8. The various stainless steels wereprocessed under conditions identical to those of Embodiment 2, andpassive films were formed thereon.

When XPS analyses of the passive films were conducted, it was discoveredthat the stainless steel having a grain number of 6 had a Cr/Fe ratiowhich was higher than that of Embodiment 2, the stainless steel having agrain number of 7 had a Cr/Fe ratio which was higher than that of thestainless steel having a grain number of 6, and furthermore, and thestainless steel having a grain number of 8 had a ratio which was higherthan that of the stainless steel having a grain number of 7.Furthermore, the thickness of the respective passive oxide films was 5nm or greater.

(Embodiment 10)

In the Embodiment 10, a stainless steel having a grain number of 5 wasemployed. Cold working was conducted prior to electrolytic polishing,and a warp of 0.3% was applied. After this, the formation of passivefilms was conducted under conditions identical to those of Embodiment 2.

When an XPS analysis of the passive film was conducted, it wasdiscovered that a stainless steel was obtained which had passive filmcharacteristics, such as Cr/Fe ratio and thickness, which were identicalto that of the stainless steel having a grain number of 8 which wasdiscussed in Embodiment 9.

Industrial Applicability

By means of the present invention, it is possible to easily and rapidlyform a passive oxide film having chromium oxide as a chief componentthereof by means of a single process, and it is thus possible to greatlyshorten processing time.

What is claimed is:
 1. A method of forming a passive oxide film havingchromium oxide as a chief component thereof, on a stainless steel havinga crystal grain number of 6 or more, wherein said passive oxide film hasa thickness of 5 nm or more and an atomic ratio value of Cr/Fe of 1 ormore at an outermost layer thereof, said method comprising;a first stepof subjecting said stainless steel to electrolytic polishing; a secondstep of baking said stainless steel in an inert gas atmosphere to removemoisture from a surface of said stainless steel; a third step of heattreating said stainless steel at a temperature within a range of 300°C.-600° C. in a gaseous atmosphere comprising hydrogen or a mixed gascontaining hydrogen and an inert gas and containing less than 4 ppm ofoxygen or less than 500 ppb of moisture.
 2. A method of forming apassive oxide film in accordance with claim 1, characterized in thatstainless steel having a crystal grain size of 8 or more is used.
 3. Amethod of forming a passive oxide film having chromium oxide as a chiefcomponent thereof, on a stainless steel having a crystal grain number of6 or more, wherein said passive oxide film has a thickness of 5 nm ormore and an atomic ratio value of Cr/Fe of 1 or more at an outermostlayer thereof, said method comprising;a first step of subjecting saidstainless steel to composite electrolytic polishing; a second step ofbaking said stainless steel in an inert gas atmosphere to removemoisture from a surface of said stainless steel; a third step of heattreating said stainless steel at a temperature within a range of 300°C.-600° C. in a gaseous atmosphere comprising hydrogen or a mixed gascontaining hydrogen and an inert gas and containing less than 4 ppm ofoxygen or less than 500 ppb of moisture.
 4. A method of forming apassive oxide film in accordance with claim 3, characterized in thatprior to electrolytic polishing, cold working having a surface reductionratio of 2% or more is conducted.
 5. A method of forming a passive oxidefilm having chromium oxide as a chief component thereof, on a stainlesssteel having a crystal grain number of 6 or more, wherein said passiveoxide film has a thickness of 5 nm or more and an atomic ratio value ofCr/Fe of 1 or more at an outermost layer thereof, said methodcomprising;a first step of subjecting said stainless steel to fluidizedabrasive polishing; a second step of baking said stainless steel in aninert gas atmosphere to remove moisture from a surface of said stainlesssteel; a third step of heat treating said stainless steel at atemperature within a range of 300° C.-600° C. in a gaseous atmospherecomprising hydrogen or a mixed gas containing hydrogen and an inert gasand containing less than 4 ppm of oxygen or less than 500 ppb ofmoisture.